High frequency loop type antenna



Aug 18, 1 942- R. A. HAMPSHIRE l 2,293,135

HIGH FREQUENCY LOOP TYPE ANTENNA Filed July l2, 1941 2 Sheets-Sheet 2 .3A Z Plas. f l@ ATTORNEY. A

Patented Aug. 18, 1,942

mon FREQUENCY Loor'rrrn maritiem' Robert A. Hampshire, New York, N. Y., assignor to Federal Telegraph Company, Newark, N. J., a corporation of California r VApplication July 12, 1941, serial No. 402,099 V9 claims. (9i. 2513"-33) This invention relates to broadcasting antennae and more particularly to high frequency broad--`- casting antennae of the loop type.A V

Broadcasting antennae have been proposed in which the antennae `elements are each less than a half-wave length long and arranged in the form of a substantiallyclosed` periphery. The separate elements are coupled end to end by capacitive coupling and alternate ones of said elements are positively energized by transmission lines coupled at spaced points on either side of the voltage nodal points, Metallic supporting struts are connected to the-voltage nodal points ofthe elements and to the supporting mast. Such types of antennae are illustrated in the copending application toA Andrew Alford, Serial No.v358',214, led September 25, 1940".' Y l i This proposed antenna had many advantages such as freedom from insulators in exposed places, and relative simplicity of construction. It was also found to have certain shortcomings. It

was vfound thatV the'radiation pattern is not precisely'circularvbut isslightly stronger in the di- This can be 'substantially'overcome' by adjusting the capacitive 'couplings to obtain substantially.i-

equal large currents in all the elements.v However, With suchadjustment the antenna does not lpresent proper impedance tol terminate the transmissionline. Adjustmentnofl the spacing of the feed line tap" improves the impedance match butleven with the best adjustment the standing waveV ratiouis still inthe order 'of 3.5 to 1. l When the loop isadjuste'dofi 'resonance the current inthe parasitic 'elements is reduced, and some current is prsent inthe supportingfstruts. My inventionconstitutes an improvement on theantennaeof the 'proposed type described above. A 4

It is a principal v'object of myinvention toprovideF an antenna for broadcasting purposes which is rigid and whichV may present such an impedance to"the feeding line as toA require 'a mini-1 mum of externalimpedancematching. Itis a'fu'rther object of my invention to provide an 'antenna which hasa substantially uniform'radiationcharacteristic in the0 plane of the antenna elements.` d

Accordingfto a feature of my invention the desired objects maybe accomplished by providing an antenna structure in which the antenna is made of radiant acting' elements each'being`short relative to the operating wave-length. The lements are arranged in the form ofa substantially t closed periphery and capacitively coupled at their 55 1n the plane of the antenna is not produced, the

ends. Each radiant acting element is divided at its center and provided with a parallel transmission line which may act as a support, short circuited at an adjusted position to secure the de- 1"fsired tuning of the loop as a whole, together with the coupling' capacities. Energy may be supplied to the loop by a transmission line coupled at a point intermediate the short circuiting bar and the associated radiant acting element of one of 16 'Zthe units. A tie line may be provided from this coupling point to alternate ones of the remaining elements so that losses in the elements may be compensated.

A better understanding of my invention and the lobjects and features thereof may be had from the 26 -With the features of my invention;

Fig. 2 is a diagrammatic showing of a preferred antenna arrangement in accordance with my invention; A

Fig. 3 is a plan View of a practical working em- 2'fbodiment of an antenna constructed in accordance with my invention;

Fig. 4 is a side elevation of a portion of the antenna shown in FigjB to illustrate the construc- Wtion; and

30 Fig'. `5 is an actual experimental radiation pattern derived from an antenna constructed in accordance with my invention. A

In Fig. l is shown an antenna arrangement formed of four radiant acting members, I, 2, 3 and V4, that isfrnembers which may be used either for 40 tively coupled together at their ends by condensers Il, I2, I3,4 I 4, A translating device I5 which may be either a transmitter or a receiver is connected by a transmission line I6 to spaced points I'I on radiant acting member 2. A substantially half-wave length line I8 isprovided for interconnecting points Il and corresponding points I9 onradiant acting member Vil. A` pair of condensers 20, 2| may be provided in transmission line I6 at av point adjacent the radiant acting member i'n order to provide the impedance match between the transmission line and the antenna.

It has been found that when an antenna unit such as shown in Fig. l is energized from one side only, a radiation pattern equal in all directions parently an attenuation of energy in all of the members due to radiation from the separatel members, Simultaneously, there is a super-imposed energizing of the member diametrically opposite the fed member. This energizing., causes a current in this opposite member which is not precisely in phase with the current in the fed member. Such phase shift in currentsmay then.

be considered to produce the unsymmetrical radiation pattern.

In order to correct the pattern it has beenY found that a' section of transmission linel sub.- stantially'a half-Wave length longv may be-connected from the tapping points on the fedmem ber to corresponding points on the diametrically opposite member. well be made other lengths'so long-as it is electrically equivalent to a half-wave length or odd multiple thereof. This tieline then delivers' power to the diametrically opposite member in an amount suiiicient to correct the loss dueto attenuation. With this arrangementv theradiation in all directions in the plane of the antenna members is substantially equal; ItV has been found that this tie-line carries a very small current compared to the current flowing in the radiant acting member.

The impedance match betweenAV the transmission line I6 and the antenna is very poor'for ordinary spaced conductor arrangements. In order to reduce this mismatch, condensersy such as 20, 2I may be provided in thel transmission line, This has been found to reduce the mismatch so that the line canV be matched` completely to the antenna.

The arrangement of Fig. 1 is'unsatisfactory in some respects due to the fact that thematching condensers must be mounted'at aA point'V adjacent the antenna and therefore, are generally relatively inaccessible. Accordingly, special precautions must be taken in order to Vassure freedom from leakage of these condensers at' the time they are installed, since atmospheric con.- ditions or sleet formations might otherwise impair their operation; Such constructions are, generally cumbersome and are not easilyV` a1'- ranged on the mast portion arrangement'.

The diiculties caused by the use ofthe' condensers may be partially overcome by utilizing` an antenna construction such as illustrated in This 1inemay equallyv theless be small compared with that obtained without this type of construction. The difficulties of providing a complete impedance match may, therefore, be greatly alleviated.

The translating device I5 is then coupled by transmission line I6 to points Il arranged along the short circuiting section at a point dependent upon the desired impedance. In most practical constructions it has been found that the coupling points II should generally be substantially at the junction point of the radiant acting parts and the transmission line section, since the short lcircuitedv section generally is not sufficient to produce an impedance match, and consequently all its impedance must be used.

In Figs- 3v and 4, is shown an actual practical construction of a loop embodying the features shown diagrammatically in Fig. 2. In this type of loop. it. is generally desirable that the antenna units be self-supporting and that they be arranged so that they may be fastened directly to a metal mast or support without the necessity of providing insulators; Accordingly, members IA, IB, 2A, 2B,` 3A, 3B, 4A, 4B, are made of metallic pipe, fastened directly to a split collar Vwhich clamps directly onto the mast.

Since parts IA, IB, 2A, 2B, 3A, 3B, 4A, 4B, occur substantially at a voltage node, they need not be provided with insulation.

The condenser elements are formed of capacitive plates IIA, IIB, I2A, I-2B, I3-A, I3B, I4A, I4B, preferably formed as parallelY discs mounted on rods IIC, IID, I2C, I2D-, I3C, I3D, IAC, I4D, adjustably arranged Within hollow membersA IB,

3552A. These plates may be clamped in adjusted Fig. 2. In this arrangement each of 'the`radiant acting members is dividedY atA its midpoint Yto form two parts IA, IB, 2A, 2B, 3A, 3B, 4A, 4B; the members being capacitively coupled by' condensers II', I2; I3, I4; Between each of the parts IA, IB, 2A, 2B, 3A, 3B, 4A, 4B, are arranged transmission line sections 5A, BA', 1A, 8A, each terminated by short circuiting bars 3l, 32, 33 and 34. These short circuiting bars are adjusted to provide a suitable terminating i'mpedance for the transmission line feeding the loop. although the adjustment of the short circuiting barsmay not, for all constructions of such a loop, give a perfect impedance match to the transmission, line. The mismatch will neverposition toV secure the proper tuning of the loop. The short circuiting bars areV made in the form of adjustable clamps BIA, 32A, j 33A, 34A, as

shown. Y

In an actual construction of a loop designedV to operate at 46 megacycles the dimension from side to side' of the loop' is' '72, the diameter of the members IA, IB, is approximatelyZl/z, theY plates IIA, I IB are about 14" inY diameter, the spacing between these plates is aboutV 8". It has been foundy that the loop can be` made quite different in size; As the loop is made larger, the radiation pattern departs from -circularity and tends tobecome square. As the loopY is made smaller, the radiation pattern remainscircular but the resonant characteristics' of the loop become similar to` thoseA of aV high quality tuned circuit. It is preferred, therefore, to' have the dimensionsfrom sideto sideof'the loopbetween and 110 electrical degrees.

In Fig; 5 is shown Ia curve of aV radiationpattern produced by experimental. measurements of the radiation. from a loop in accordance with" my invention. This loop was made withY theA same wave lengths proportions as those given for the particular" example outlined' above; but scaled down for operation ati megacycles.'

While I have described'the princi-pal'features of my invention in connection with the preferred form of the loop, it is to be understood that departures may be made therefrom, without de'- parting fromv the spirit of myV invention. For example, the loop need not be mades'quare'in Vform but the members may be bent-into any desired regular Vgeometrical configuration.' 'Fur` thermore, all points of supportr occur at substantial voltage nodes, so that direct connections can be made withoutv producingv large currents in the supporting structure and consequent losses- Also, although it' ispreferable to makeVV the antenna self-supporting the principles-of my invention apply as well to any antenna structure even if separate supporting means are required for the radiated members.

What is claimed is:

1. An antenna unit comprising four radiant acting members each being short with respect to a half Wavelength at the operating frequency, said members being arranged to forma substantially closed periphery, capacitive reactance means coupling together adjacent ends of said members, a translating device, a transmission line interconnecting said translating device and one of said radiant acting members, and a substantially half wavelength line interconnecting the junction points of said transmission line and said one radiant acting member and corresponding points on the oppositely positioned member.

2. An antenna unit according to claim 1 further comprising capacitive reactance means in series with said transmission line adjacent said one radiant acting member.

3. An antenna unit according to claim 1 in which said radiant acting members are divided substantially at their mid-points into two parts further comprising a short circuited transmission line section connecting together said two parts.

4:. An antenna unit comprising a plurality of radiant acting members each being short with respect to a half wavelength at the operating frequency, said members being arranged in the form of a substantially closed periphery, capacitive reactance means coupling together adjacent ends of said members, each radiant acting member being formed of a pair of substantially equal length elements, a section of parallel transmission line extending from the adjacent ends of ea-ch of said pairs of elements, a short-circuiting bar arranged at a predetermined point along each of said transmission line sections, a translating device, and means for coupling said translating device to a point on one of said transmission line sections intermediate said elements and said short-circuiting bar.

5. An antenna unit according to claim 4 wherein said transmission line sections are made of relatively heavy conductive material, whereby they may serve to support said elements, further comprising a metallic supporting structure, and means for rigidly xing said transmission line sections to said metallic supporting structure.

6. An antenna unit according to claim 4 further comprising conductive transmission means interconnecting said intermediate point and a similar intermediate point on another of said transmission line sections.

'7. An antenna unit comprising a four of radiant acting members each being short with respect to a half wavelength at the operating frequency, said members being arranged vin the form of a substantially closed periphery, capacitive reactance means coupling together adjacent ends of said members', each radiant acting member being formed of a pair of substantially equal length elements, section of parallel transmission line extending from the adjacent ends of each of said pairs of elements, a short'l circuiting bar arranged at a predetermined point along each of said transmission line sections, a translating device, and means for coupling said translating device to a point on one of said transmission line sections intermediate said elements and said short circuiting bar.

8. An antenna unit according to claim 7 further comprising a, conductive transmission means interconnecting said intermediate point and a similar intermediate point on the oppositely positioned transmission line section.

9. An antenna unit according to claim 7 further comprising capacitive reactance means in series with said transmission line adjacent said one radiant acting member.

ROBERT A. HAMPSHIRE. 

